1. Field of the Invention
The present invention relates to an image pickup device having a frame-size memory with an electronic shutter function.
2. Related Background Art
In recent years there has been proposed a sensor of FGA (floating gate array) type. FIG. 1 is a block diagram showing an example of an area sensor of FGA type mentioned above. There are provided a photoelectric conversion unit 10 in which a plurality of the photoelectric conversion cells 10C are arranged in a matrix shape; vertical address lines 10V for designating the vertical address; signal reading lines 10S for reading the photoelectric conversion cells of a row designated by the vertical address line 10V; a resetting circuit 12 for resetting the photoelectrically converted signals in the photoelectric conversion circuit 10; a clamping circuit 14; a line memory 16; an output signal line 18 for reading the signals stored in the line memory 16 in succession in the horizontal direction; a horizontal shift register 20 for designating the signal reading position of the line memory 16; switches 22 controlled by said shift register 20; an output buffer 24 of a high input impedance; and an output terminal 26.
An address decoder 28 selectively activates the vertical address lines 10 of the photoelectric conversion unit 10. A decoder driving circuit 30 designates the vertical address line to be activated by the address decoder 28, according to vertical address data D.sub.VA. The address decoder 28 sends a signal .phi..sub.H to the vertical address line 10V for a row for which the signal reading or resetting is to be conducted, and a signal .phi..sub.L to other vertical address lines 10V. 32 indicates coupling capacitors.
There are further shown resetting FET's 12T; clamping FET's 14T; switching FET's 16T for storing the output signals of the clamping circuit 14 in the line memory 16; memory capacitors 16M; a reset pulse .phi..sub.G for controlling the resetting FET's 12T of the resetting circuit 12; a clamp voltage V.sub.R ; a clamping pulse .phi..sub.C ; a sample-hold clock .phi..sub.SH for controlling the FET's 16T; a start pulse STATH for activating the shift register 20; and a shift pulse .phi..sub.S for the shift register 20.
The line memory 16 and the clamping circuit 14 are shielded from external light.
FIG. 2 shows the structure of a photoelectric conversion cell 10C, and FIG. 3 shows the function timing thereof. A pulse source 34 for generating clock signals .phi..sub.H, .phi..sub.L corresponds to the address decoder 28 shown in FIG. 1. An N-channel junction FET 36 with a floating gate G constitutes a light-receiving element, and is connected to the vertical address line 10V through a capacitor 38. The drain of the FET 36 is connected to a DC power source V.sub.DD, and the source S is connected to a resetting FET 12T of the resetting circuit 12, and also to a signal reading line 10S. Referring to FIG. 3, when the vertical address line 10V is shifted to an H-level by the pulse source 34 in a period from t.sub.1 to t.sub.2, the gate-drain junction of the FET 36 is biased in the forward direction, whereby the capacitor 38 is pre-charged. If the gate area of the FET 36 is not irradiated with light, said gate remains in the fully inversely biased state (broken line in FIG. 3) in a period t.sub.2 - t.sub.3. If the gate area is irradiated with light, the capacitor 38 is gradually discharged by the photo-induced charge, whereby the gate potential is elevated (solid line in FIG. 3). Since the source potential of the FET 36 follows the gate potential, a voltage corresponding to the intensity of incident light can be obtained on the signal readout line 10S.
FIG. 4 shows the timing of an image taking operation of the image pickup device shown in FIG. 1. A horizontal blanking period is started by a horizontal blanking signal HBLK, and a vertical address D.sub.VA is supplied to the decoder driving circuit 30 at a time t.sub.1, whereby a signal .phi..sub.H is supplied in succession to the vertical address lines 10V, while a signal .phi..sub.L is given to other vertical address lines. When the signal .phi..sub.L assumes the L-level at the time t.sub.1, all the FET's 36 of the photoelectric conversion cells 10C receiving said signal are turned off, so that only the signals of the photoelectric conversion cells 10C connected to the designated vertical address line 10V are supplied to the signal readout line 10S. As the clamping pulse .phi..sub.C and the sample-hold pulse .phi..sub.SH are both at the H-level in the period from t.sub.1 to t.sub.2, the capacitors 16M of the line memory 16 are reset to a reference potential V.sub.R. The clamping FET's 14T are opened at the time t.sub.3. When the clock signal .phi..sub.H assumes the H-level in a period from t.sub.4 to t.sub.5, the capacitors 38 are pre-charged, wherein the charges across the coupling capacitors 32 are proportional to the amounts of charge generated by photo-induced voltages in the photoelectric conversion cells 10C. Said voltages of the coupling capacitors 32 are transferred to and stored in the capacitors 16M in response to the H-level of the signal .phi..sub.SH in a period from the time t.sub.6 to t.sub.7.
A resetting operation for controlling the charge accumulating time is conducted in a period from t.sub.9 to t.sub.11. The vertical line address to be reset is designated at the time t.sub.9, and the charges of the designated line are reset in a period t.sub.10 -t.sub.11. The charge accumulation is conducted from the resetting to the next signal reading from this line. The horizontal shift register 20 is driven by the shift pulses .phi..sub.S starting from a time t.sub.13, whereby the signals stored in the capacitors 16M are transferred, in succession, to the output line 18, and are released to the output terminal 26 through the buffer 24.
A conceivable application of the above-explained area sensor of the FGA type is in the recently commercialized electronic still camera. In the electronic still camera, a frame recording of an image signal is made possible for improving the quality of the recorded image. However, the output of such an image pickup device is recorded in the form of an odd field and an even field on a magnetic disk. The timing of photoelectric conversion for the odd field is different by 1/60 seconds from that for the even field. However, in case of a moving object, the position of the object is different between the different fields, so that a satisfactory image cannot be obtained as a frame image. Such a problem is not limited to the image pickup device of the FGA type but commonly exists in image pickup devices of the MOS, or the SIT type, and in XY-addressed sensors composed of bipolar transistors.
Also such a sensor may be used in a hand-held video cassette recorder combined with a camera, or an electronic still camera. Such equipment usually has a reproducing function, and the reproducing circuit has been complicated by the use of a 1H delay device for example in a dropout compensating circuit or a circuit for multiplexing sequential color difference signals.
Also in the conventional equipment, a 1H delay line is also required for correcting the vertical aperture, and the circuit becomes inevitably large for this reason. Furthermore, for obtaining an uncorrelated component with a subtractor, gain control has to be achieved for example with resistors. Furthermore, in case of interlace reading, vertical aperture correction has conventionally been conducted between two horizontal signals across a horizontal line. However, if the vertical aperture correction is conducted between mutually neighboring horizontal signal lines, there is required a 1V delay line such as a field memory, instead of the 1H delay line, so that the circuit becomes even larger.
Also in the conventional signal charge reading with two rows at a time, all the charges of the pixels of the solid-state image pickup device are destructively read by two rows at a time for obtaining a still image of a field, so that the image of the second field cannot be obtained. Stated differently, for obtaining a still image of a frame, there is required an image pickup device having a doubled resolution in the vertical direction, which is extremely expensive and is not usable for a consumer electronic appliance such as the electronic still camera.